Experiments, techniques, links, and activities for an Elementary School Science Club.

Wednesday, November 30, 2016

Quicksand and other crazy polymers - Session by Shannon Stott

Education goals:

-Introduce the concept of unique liquids or polymers. Specifically focusing on liquids that sometimes behave like a solid and sometimes they behave like a liquid.

-Get the children to think about how materials and liquids can change their properties in response to external factors such as temperature and physical force.

-Introduce the idea of solutions and composite materials (mixing different components together can dramatically change the substance and composition).

-Outside of having fun and making a mess, introduce them to scientists and jobs that involve this type of work.

Outline of lesson:

-Talk about quicksand.

-what is quicksand? (show the picture of the child and mother in quicksand at a beach)

Answer: a mixture of sand and water at just the right ratios where the sand behaves less like a solid and more like a liquid

-has anyone heard of it?

-has anyone been in it?

-what causes it?

Answer: Rapid flooding of an area, earthquakes, tides on the beach

End with: Did you know that quicksand is a POLYMER? A polymer is a special word for just a fancy kind of liquid. There are long strands of molecules that all look the same that make up polymers. The water and polymer combined allows it to act in crazy ways. Sometimes it is a liquid and sometimes it is a solid. Should we make our own polymer that acts like quicksand?

Hands-on portion (Part I: Cornstarch Quicksand)

Break students in groups of three.

Supplies for each group:

-Two large bowls (one for water and one for the ‘quicksand’)

-Cornstarch

-Measuring cups

-Wooden sticks for mixing.

-Newspapers to place on the work surface.

Important notes:

(1) Starting off with too much cornstarch makes it incredibly difficult to mix. You need to go slowly and gradually increase the cornstarch / water ratio. A good ratio to start with is 1:1 ( water: cornstarch), building up your volume so that you have plenty of ‘quicksand’ to put your hands and fingers into. The final concentration is typically between 1:4 – 1:10 (water to cornstarch).

(2)Never pour the cornstarch ‘quicksand’ down the sink as it will clog the drains. Place it in a Ziploc bag or dispose of it in the trash using some other means.

Protocol.

Designate a ‘starting’ student in the group. Have them add 1/3 cup cornstarch to the empty bowl. Then have them add ¼ cup water (trying not to mix the cups, so that you don’t get a bunch of water in your cornstarch). Mix.

Move to the next student and have them repeat. If the mixture gets too much cornstarch at the beginning, just add a bit more water.

Keep adding more cornstarch and water, gradually increasing the amount of cornstarch. Eventually, you will just be adding cornstarch in small amounts and mixing it in.

Once you have achieved an “ooblek’ consistency (I just mix it with my hands to test it--- I also punch it to make sure we are ‘there’), remove the excess supplies (cornstarch, water, etc) and have the groups focus on the properties of next phase (interacting with the ‘quicksand’)

Actions/Discussion

Ask the students to place their fingers in the ‘quicksand’. What does it feel like? Is it a liquid or a solid? When they stir the ‘quicksand’ with their finger, is it easier or harder to mix it if they try to stir it quickly? (let each student in the group try, one at a time). When we apply force (such as mixing it faster) to the liquid, it acts more like a solid.

Now we want to make the ‘quicksand’ act even more like a solid. To do this, we can ‘punch’ the liquid. Demonstrate this. Have the students do it.

Now let us pretend like our hand is ‘stuck’ in the quicksand. Let our fist and fingers ‘sink’ in and then rapidly try to pull it out (like you are panicking in quicksand). What happens? (the whole bowl lifts up--- make sure the students are putting as much of their hand ‘under’ the quicksand and encourage them to bend their fingers).

Depending on their focus, you can state that this is a ‘non-newtonian’ fluid, or a fluid that behaves differently than Isaac Newton first modeled. Rather than changing with temperature, our quick sand changes to force.

Ask if they want to see a video of someone ‘walking on a solution similar to the one we just made’. Mention that they made a giant bathtub full of cornstarch quicksand and tried to see if someone could walk on top if it withouth sinking in. (Play Ellen video through smart board).

When you apply gentle forces, the water has time to escape from the cornstarch ‘polymer’ and it acts more like a liquid.

When you apply fast, strong forces, the water cannot escape and gets trapped, causing the mixture to act like a solid.

Talk about other polymers / liquids:

Toothpaste: is it a solid or a liquid (does it drip out of the tube when you hold it upside down? (no, it acts like a solid). Yet, when you squeeze it (apply force), it does the opposite of quicksand and turns into a liquid).

Ketchup: it is similar to toothpaste. You need to apply force (shaking / tapping) to get it out of the bottle. Force turns it into a liquid.

Blood: The blood in our body adjusts how it ‘flows’ through different compartments / blood vessels based on physical forces.

Water: water is very different than the above liquids. It is a Newtonian fluid, meaning it does not change when you apply physical forces.

Discuss how to escape quicksand:

Don’t panic! If you move too fast, it will behave like a solid and trap you! Just like your hand was trapped before.

Get rid of extra weight, like backpacks.

Try to lay on your back (like swimming) and then slowly raise your legs up.

Once on top of the sand, either swim, or roll quickly to the ‘safe sand’ part.

Discuss famous polymer scientists.

Show picture of Stephanie Kolwek. For adults: she was a scientist who worked at Dupont and invented Kevlar (she has lots of patents on different materials she invented). For kids: This is a picture of Stephanie Kolwek. She is a scientist that invented something really important. She invented a POLYMER that is called Kevlar. Does anyone know what Kevlar is? What about a bullet proof vest? Does anyone know what that is? The vest is made out of her special polymer that stops bullets! Isn’t that so cool. (then read the quote about how her ‘liquid’ has saved thousands of lives.).

Discuss the 11 year old scientist / inventor, Peyton Robertson. He invented a material that can be used in ‘sandless sandbags’. Ask if they know what a sandbag is. Mention how they are used to stop floods and water leaking but they are very heavy because sand typically fills them. Peyton invented a polymer that expands and gets heavier when wet, so you can bring it (light) to the flood and then it expands with the water to create the heavy sandbag.

-Food coloring (Leader holds onto it, doing one group at a time, about 3-5 drops)

-Wooden sticks for mixing.

-1/4 cup Sta Flo Liquid Starch

-Newspapers to place on the work surface.

Important notes:

-Once you mix the Sta Flo liquid starch into the mixture, it will cross-link (turn it into a solid) the glue. Thus, if you want to add color, you need to add it right to the glue.

-Unlike Monday, we are using Sta Flo to cross link (we used Borax, a laundry detergent on Monday). Thus, we do NOT need to add water to the glue this time.

Protocol.

Remove the orange cap to the Glue. Add the whole bottle of glue to the mixing bowl.

Add food coloring to the glue (3-5 drops). Have students mix in the color, using care to not get their clothes dirty. Do not move to the next step until the color is mixed in.

While they are mixing in the food coloring, add the StaFlo into the Elmer’s bottles (or use a 4oz measuring cup and walk around adding the starch to each student’s mixture.

As they mix in the starch, it will ‘cross link’ the polymers, turning it into a solid-like substance. After they do the mixing, let it sit for five minutes (Borax is an immediate reaction; whereas StaFlo takes some time). This would be a good time to distribute the containers to bring the putty home.

After the five minutes of sitting, they can remove the putty, place it on a paper towel and ‘knead’ it for 5-10 minutes.

Done! They can split the putty into two and play with it.

Actions/Discussion

Ask the students to slowly stretch their putty. What happens? Is it a liquid or a solid?

Ask them to put it back into a ball and then pull it apart quickly. What happens? It is a liquid or a solid?

Have them put it back into a ball and see if it can bounce on the table top.

If time, they can let the putty ‘hang’ from their fingers and a slow, ‘slime’ like behavior will start to happen. It is ‘flowing’ like a slow liquid.

Discuss how it is similar to the cornstarch (acts like a solid or liquid depending on how you apply force) and how it is different (more solid, this is because we ‘cross-linked’ or bound together some of the particles with the starch).